Over the past several years, optical sensors have proven to have broad applicability in the remote measurement of physical, chemical, and biological phenomena. Such sensors can be used to measure a wide range of variables including but not limited to temperature, pressure, force, flow, radiation, liquid level, pH, displacement, humidity, vibration, strain, rotation, velocity, magnetic and electric fields, acceleration, acoustic fields, and to detect the presence of and identify one or more chemical species. In addition to this wide range of potential uses, optical sensors have a number of other benefits. For example, they are often small, compact, light and have a longer lifetime than other types of sensors. Optical sensors also tend to be immune to electromagnetic interference, can be electrically isolated, and often have high sensitivity. As a result, these sensors are not only replacing conventional sensors in many areas in science, engineering, and medicine but researchers are beginning to create new kinds of sensors that have unique properties. Since remote sensors do not require direct contact with the measured target nor do they require sampling of any chemical, they provide the ability to scan large areas and volumes in a short period of time.
There is utility and demand for optical sensors in an array of industries. Environmental and atmospheric monitoring, earth and space sciences applications, industrial chemical processing and biotechnology, law enforcement, digital imaging, scanning, and printing are a few examples. In addition to these more established uses, there is an ever increasing need for devices and methods capable of early, passive, and remote detection of dangerous gases and other substances, particularly for security applications, but also for safety. Recently, the need for these systems has been heightened by the spread of chemical warfare technology around the world and the increasing number of acts of global terrorism, for example, threats against chemical plants can adversely impact large population centers in the vicinity of such plants. Indeed, the potential release of dangerous substances is now a serious concern not only for the military, but domestically as well. The ability of remote sensors to probe large volumes quickly and without actually entering a chemical cloud provides obvious benefits to first responder and law enforcement agencies.
The development of passive remote detectors has also been driven by other factors, such as the growing concern regarding the effects of industrial and vehicular emissions as well as other forms of environmental pollution like pesticide over application. Remote detectors are also needed to monitor and study long-term trends as well as to identify and provide warnings regarding day-to-day environmental problems that may affect the health of local and global populations.
Although many remote sensors, e.g., lidar based systems and Fourier transform interferometers, can meet most of these detection objectives, they are complex, expensive, large and heavy. There is a strong and unmet need for low-cost, highly portable and robust devices. For example, in many domestic security applications it is desired that each police car or fire truck include a sensitive remote sensor of chemicals that can be hand carried by its crew to the site of potential incidents and operated without actually endangering its operator. Similarly, public buildings require advanced warning to protect occupants against chemical threats released indoors or outdoors. The cost of such a sensor must meet the needs of local government authorities and the level of complexity and training must meet the ability of police officers or firefighters.
In some optical remote sensing devices, designed to provide low-cost, robust and simple systems (e.g., differential absorption radiometer—DAR), a method of collecting, modulating, and distributing (i.e., multiplexing) light to multiple detectors is required. Here light is used to describe all types of electromagnetic radiation, including but not limited to x-rays, ultraviolet, visible, infrared and microwave radiation. In such applications, multiple detectors are often fitted with different filters (e.g., bandpass, notch, long pass, short pass, diffractive, or polarizers), for spectral analysis of a target. Further, many types of detectors must receive an amplitude modulated signal to optimize their response (e.g., pyroelectric detectors) or to reduce noise and enhance detectivity by allowing for detection by demodulation in the frequency domain. While multiplexing,. i.e. providing a coincident field of view (FOV) for multiple detectors, may be achieved with the use of multiple lenses, split lenses, or various configurations of polarizers or split mirror, each of these techniques requires precise alignment of the optical components for each detector thereby increasing complexity and cost and making the system susceptible to misalignment by vibration, mild shocks, temperature variations, acoustics etc. thereby rendering them inadequate for their intended applications. Further, these techniques do not provide modulation of the collected radiation and reduce the available radiation by 1/N where N is the number of detectors. Radiation modulation is typically achieved by either spinning wheel or tuning fork choppers, or polarization modulators which must then be added to the already complex multiplexed system. Both modulation techniques block radiation for a certain fraction of the modulation cycle, thereby reducing the collection and light management efficiency. Typical mechanical modulators use a 50% duty cycle (closed 50% of the time).
Thus, there is a need in the art for a simple, robust and low-cost method and system for simultaneous multiplexing and modulation that may include placing multiple detectors behind a large single lens, distributing, modulating, aperturing, and spectrally analyzing the collected optical signal among the detectors by sequentially illuminating the detectors through a masking system and spectrally resolving components such as bandpass, notch, long pass, short pass, diffractive, or polarizers. In this case, detectors are illuminated at full collection intensity, but for a fraction of the duty cycle, allowing the signal to be recorded only when an individual detector is illuminated by radiation from the target while avoiding noise from being recorded when that detector is not illuminated. Also, by densely packing the detectors in linear, circular, or ring type arrays (thus reducing space between adjacent detectors), sequential illumination can reduce radiation loss only to periods when radiation falls within the small spaces between the adjacent detectors.